In power electronics, there are three types of switches
Here the ON and OFF of switch cannot be controlled.
Like the diode, the diode gets ON when forward biased and it turns OFF when reverse biased.
Here the ON or OFF can be controlled, but the other one cannot.
Like SCR-silicon controlled rectifier. It turns ON when it is
Triggered by gate
After the SCR is forward biased, you can turn it ON by triggering through gate terminal.
So it is Semicontrolled Switch.
Here both the ON and OFF can be controlled.
Example-power BJT, power MOSFET or IGBT. We turn on by applying positive base signal and turn off by applying negative base signal.